1. Field of the Invention
The invention relates to a piezoelectric thin-film element, a piezoelectric sensor and a vibration generator.
2. Description of the Related Art
Piezoelectric materials are processed into various piezoelectric elements according to different purposes. The piezoelectric materials are widely used as functional electronic components, e.g., actuators for causing deformation of a piezoelectric element using applied voltage, piezoelectric sensors generating voltage by the deformation of the piezoelectric element, or vibration generators for generating power using vibration, etc., applied to the piezoelectric element.
Pb-containing perovskite-type ferroelectric materials having excellent piezoelectric properties and called PZT (lead zirconium titanate) are widely used as piezoelectric elements which are used for actuators and piezoelectric sensors, etc. Conventionally, a piezoelectric material layer of such a piezoelectric element is formed by a sintering process in which PZT as an oxide is sintered.
Currently, with the progress in downsizing and high performance of various electronic components, piezoelectric elements are also required to be downsized and to have higher performance. However, when the piezoelectric material layer of the piezoelectric element is formed by the sintering process, grains of crystal constituting the piezoelectric material layer become large and this causes a problem that variation and degradation in characteristics become pronounced in case that the piezoelectric material layer is formed to have a thickness of about 10 μm.
In order to solve such a problem, a method of forming a piezoelectric thin-film layer using thin-film technology has been studied as an alternative to the sintering process. For example, a piezoelectric thin-film element using a PZT piezoelectric thin-film layer formed on a Si substrate by a sputtering method has been put to practical use as a piezoelectric thin-film element for actuator or angular velocity sensor of high-speed high-definition inkjet printer head.
On the other hand, the piezoelectric thin-film element having the PZT piezoelectric thin-film layer contains about 60 to 70% of Pb. Therefore, in view of impact on living bodies and environment, it is desired to develop lead-free piezoelectric thin-film elements.
Some of such lead-free piezoelectric thin-film elements use a piezoelectric thin-film layer formed of KNN (potassium sodium niobate) represented by (K1-xNax)NbO3 (0<x<1) (see, e.g., JP-A-2007-184513 and JP-A-2008-159807). This KNN piezoelectric thin-film layer has a perovskite structure and shows promise as a highly possible lead-free piezoelectric thin-film element.
Meanwhile, it has been reported that a KNN piezoelectric thin-film element on a Si substrate by the sputtering method achieved a piezoelectric constant of d31=−100 pm/V which is a practical level of the layer preferentially oriented to a (001) plane direction (Non-Patent Literature: Japanese Journal of Applied Physics 50(2011)041503).